Researchers say there is genetic evidence to show that Africa's elephants form two separate species.

The difference between the two is more than half the genetic distinction between African and Asian elephants.

The researchers say they found very little evidence of interbreeding between the two African species.

They believe their discovery will have implications for the conservation of African elephant populations.

The researchers are from the National Cancer Institute, Maryland, US, and the Mpala Research Center in Kenya. They report their findings in the magazine Science.

Using biopsy samples collected by dart from 195 free-ranging elephants in 11 African countries, they tried to see if there was a genetic difference between animals that live on the savannah and those that spend their time in the forests.

Visibly distinct

They compared what they found with similar material from seven Asian elephants.

The different physical appearance of the two African groups has been acknowledged for many years.

Forest elephants appear genetically distinct

Forest elephants are smaller and have straighter and thinner tusks than their savannah cousins, and they have rounded ears and distinct skull morphology.

But although they have sometimes been regarded as a subspecies, the researchers say, the forest elephants' "degree of distinctiveness and of hybridisation with savannah elephants has been controversial and often ignored".

The researchers compared the DNA variation in four nuclear genes, and say their analysis suggested that genetic distinctions between forest and savannah elephants correspond to 58% of the difference in the same genes between African and Asian elephants.

Another finding was that the savannah populations in southern, eastern and north central Africa, although widely separated, were genetically indistinguishable, while the forest animals showed more genetic diversity.

The researchers write: "Each savannah population was genetically closer to every other savannah population than to any of the forest populations, even in cases where the forest population was geographically closer."

They suggest also that there are in practice strong reasons that discourage interbreeding between the two groups.

Long separation

They write: "The paucity of gene introgression between forest and savannah populations even near regions of potential physical contact suggests that hybridisation in nature is rare and perhaps minimised by behavioural or physiological reinforcement."

The researchers believe that "a long period of adaptive evolution (estimated at 2.63 million years) separated the savannah and forest elephant lineages".

They say their findings are "cogent indicators that there should be species-level recognition for Loxodonta africana, the African savannah elephant, and Loxodonta cyclotis, the African forest elephant".

Asian elephants are a distinct species

They conclude: "Given the rapid depletion of both forest and savannah elephant numbers in the past century and the ongoing destruction of their habitats, the conservation implications of recognition and species-level management of these distinct taxa are considerable."

There are several approaches to elephant conservation in Africa. Kenya has attacked the ivory trade with extreme rigour, shooting ivory poachers in order to protect the elephants.

Rethink possible

The policy has worked, in the sense that elephant numbers in Kenya are now climbing again after years of steep decline.

Across the continent, poaching is thought to have more than halved the elephant population within two or three decades to a low point of about 600,000 animals ten years ago.

But several southern African countries say they have too many elephants, and have been culling them to reduce their numbers.

If the researchers are right in saying there are two distinct species, both approaches may need to be rethought.